A stable community is one where all the species and environmental factors…

The idea

A stable community has all species and environmental factors in balance, so population sizes stay fairly constant over time. Stability does not mean nothing changes; it means changes are small or self-correcting. For example, if a predator's population rises, prey numbers may fall, which reduces food for the predator and its numbers fall again. Environmental factors such as temperature, water and light also remain suitable. A stable community therefore resists small disturbances and maintains biodiversity. If a major factor changes, such as a new predator or a drought, the balance can be lost and populations may change significantly.

What earns the marks

• States that a stable community has species and environmental factors in balance.
• Explains that population sizes remain fairly constant rather than changing dramatically.
• Recognises that stability involves small fluctuations that are corrected by negative feedback.
• Gives an example of a factor, such as food supply or predation, that helps maintain balance.
• Explains that a major environmental change can destabilise a community and alter population sizes.

Common mistakes and how to put them right

• Thinking stable means no change at all.

  Put it right by explaining that populations fluctuate slightly but remain fairly constant.

• Assuming only living factors matter.

  Put it right by including environmental factors such as temperature, light and water.

• Believing a stable community cannot be affected by change.

  Put it right by stating that large disturbances can upset the balance.

Exam tips

• Use the phrase fairly constant when describing population sizes in a stable community.
• Refer to both species and environmental factors to cover all parts of the definition.
• When interpreting data, look for small fluctuations around a mean rather than a perfectly flat line.